WLAN (wireless local area networks) systems are one of the popular forms of communication systems. WLAN systems are governed by 802.11 standards. As known, the standard 802.11a is the WLAN standard for 5 GHz spectrum transmissions, based on Orthogonal Frequency Division Multiplexing (OFDM). It is noted that the systems governed by the 802.11b standard for 2.4 GHz spectrum are based on Direct Sequence Spread Spectrum/Complementary Code Keying (DSSS/CCK). It is also noted that the standard 802.11g is an enhancement over the standard 802.11b operating in 2.4 GHz band. The standard 802.11g supports transmissions of both 802.11a and 802.11b frames in 2.4 GHz band giving a maximum data rate of 54 Mbps.
A communication signal communicated over a WLAN system is typically encoded in a plurality of frames, each of the frames may include a plurality of symbols. The standard 802.11b may provide a training period of 56 microseconds for short preamble mode. The training period is a time period in which the system has to detect the signal type and/or determine characteristics of the communication signal received by the system. During the training period the receiver requires to perform a number of operations, these operations may include: determining gain—Automatic Gain Control (AGC); selecting best antenna—Antenna Diversity (AD); acquiring and detecting symbol boundaries—frame synchronization; estimating and correcting Carrier Frequency Offset (CFO) and Sampling Frequency Offset (SFO); estimating channel and equalizer coefficients, etc.
Typically for a system operating according to 802.11b standard, locking AGC requires about 8-9 micro seconds, acquisition and boundary detection require about 12 microseconds, channel and rake estimation require about 15 microseconds, coarse frequency estimation requires about 11 microseconds. So, for a single antenna system, total time required for synchronization and equalization related operations is about 47 microseconds. Of the available 56 microseconds of training period, 7 microseconds is required to synchronize the descrambler. Hence, due to scarcity of time, antenna diversity and/or other operations are often compromised in respect of quality.
In a system operating according to 802.11b standard, the data bits are modulated, spreaded to form symbols and transmitted. At the receiver end, it may be required to detect the presence of valid packet and then find the symbol boundary before demodulating the data. Further it is likely, that the carrier and sampling clocks at the transmitter end and at the receiver end will not be synchronized resulting in frequency offset and accordingly, it may be required to determine and correct the frequency error.
The spreading code used during the training period is 11-chip Barker sequence in 802.11b systems. In the subsequent discussion, for the purpose of explanation, Barker correlated signals are discussed.
FIG. 1 shows a typical block diagram 100 for acquisition and boundary detection and coarse frequency estimation. The received signal is passed through a matched filter 110 where a sliding Barker correlation is performed on the received signal. An acquisition and boundary detection block 120 determines the presence of valid packet and the symbol boundary using the output of matched filter. A Barker correlator block 130 begins the correlation at the detected symbol boundary. The Barker correlator block 130 output is used to compute the coarse frequency error.
Various steps performed during the training period according to the standards 802.11a and 802.11b have been discussed independently in literature. In this context, reference may be had to the publication of Nov. 18, 2003 titled “Improving Efficiency When Detecting WLAN Preambles,” authored by Richard Williams, and published in Communications Design. Other publications which may be referred to include: Timothy M. Schmidle and Donald C. Cox, Robust Frequency and Timing Synchronization for OFDM, IEEE Transactions on Communications, Vol. 45, No. 12, December 1997; and J. Heiskala and J. Terry, OFDM Wireless LANs: A Theoretical and Practical Guide, SAMS Publishing, 2002.